The present invention relates to mobile radio and, more particularly, to mobile data communications.
A mobile data communications channel is referred to as an "impaired channel" since it is affected by a number of channel impairments like Rayleigh fading, co-channel interference, etc., that increase the error rate and, thus, decade the overall performance of the mobile data connection. One form of mitigating the affects of an impaired channel is to simply apply land-line modem standards to the mobile radio environment.
In the lid-line modem arena, International Telephone and Telegraph Consultative Committee (CCITT) standard V.32bis is a representative modulation standard. In V.32bis, during the "start-up mode," or "connection phase," both modems, or endpoints, establish the data connection, e.g., perform a "handshaking" sequence to establish the modulation standard error control, and the data rate. After the start-up mode, the "data mode," or "communications phase" is entered in which data, or information, is exchanged between the two modems over the mobile data communication channel. One of the features of V.32bis is the ability to sequentially "autorate" upwards or downwards between 4800 bits per second ((bps) and 14400 bps during either the start-up mode or the data mode as a function of the error characteristics of the communications channel. For example, if a land-line data connection is initially established between two modems at 14400 bps and the error rate subsequently increases beyond a predetermined threshold, one, or both, of the modems will negotiate a data rate change down to the next lowest data rate. In this example, the data rate would change from 14400 bps to 9600 bps. At the latter data rate, information is then transmitted for a period of time to monitor the resulting error rate. If the error rate is still above a predetermined threshold, one or both, of the modems will, again, negotiate a data rate change down to 4800 bps. On the other hand, if the error rate decreases for a period of time while transmitting data 9600 bps, one, or both, of the modems will negotiate a data rate change to the next highest data rate, i.e., from 9600 bps to 14400 bps. In either event, the autorate feature takes advantage of the fact that the error rate of a communications channel is typically also a function of the data rate, i.e., the higher the data rate, the likelihood of an error increases.
Thus, in the context of a mobile data communications channel, the land-line autorate feature is used to assist in mitigating the affect of an impaired channel on the communication of information between two modems. If transmitting at 14400 bps and the error rate increases beyond a predetermined threshold, one, or both, of the modems will negotiate a data rate change to 9600 bps in an attempt to improve the performance of the mobile data communications channel. Unfortunately, if the last permissible data rate is reached, i.e., 4800 bps, under the V.32bis modulation standard, one, or both, of the modems simply disconnect if the data communications channel continues to be severely impaired.
Another alternative approach is represented by the MNP-10 protocol developed by Microcom Inc. specifically for mobile data communications. In this approach, the MNP-10 protocol initiates a data connection at 1200 bps and allows sequential autorating at all data rates between 1200 bps and 14400 bps as a function of the error rate of the mobile data communications channel. For example, a modem running the MNP-10 protocol can increase the data rate in a sequence of steps, i.e., from 1200 bps to 2400 bps to 4800 bps to 7200 bps to 9600 bps to 12000 bps to 14400 bps if the resulting error rates are low enough; or the modem can decrease the data rate in a sequence of steps in the opposite direction if the error rates are high enough. For example, if the data rate is 14400 bps, i.e., MNP-10 provides for a fallback ability, in sequence, from 14400 bps to 12000 bps to 9600 bps to 7200 bps to 4800 bps to 2400 bps to 1200 bps until an acceptable error rate is reached. If the data rate finally reaches 1200 bps and the error rate is still too high, one, or both, of the modems will disconnect. During start-up mode, an MNP-10 modem can be strapped to initiate the data connection at either 1200 bps or 4800 bps. However, like V.32bis, there is no autorating below 4800 bps during the start-up mode, so that if there are problems in just establishing the data connection, one, or both, of the MNP-10 modems will disconnect.